1. Technical Field
The present invention relates to a device suitable for driving a stepping motor used to feed a document or recording paper in a sub-scanning direction in a facsimile machine, a scanner, a printer or the like.
2. Background Art
In a facsimile machine, generally a density of sub-scanning lines in the document scanning is 3.75 lines/mm, 7.7 lines/mm or 15.4 lines/mm. Scanning or recording at a 3.75 lines/mm density is called "normal mode", that at a 7.7 lines/mm density is called "fine mode" and that at a 15.4 lines/mm density is called "superfine mode" in the art. The sub-scanning line density of the fine mode is twice that of the normal mode. In other words, an amount of document feed between lines in the fine mode is half that in the normal mode.
A stepping motor is considered the best device for feeding the document or recording sheet in a facsimile machine. This is because the stepping motor can be rotated at a desired step and because a half step movement becomes possible by switching a two-phase excitation to a one-two phase excitation. For example, the stepping motor can feed the document or recording sheet at one step of the two-phase excitation in the normal mode whereas it can feed the same at a half step of one-two phase excitation in the fine mode. A motor drive device is connected with the stepping motor and a motor drive circuit of the motor drive device excites the stepping motor. The motor drive device two-phase excites the stepping motor in the normal mode and one-two phase excites the same in the fine mode on an instruction from a CPU of the facsimile machine.
When the conventional motor drive device performs the one-two phase excitation of the motor, a hardware structure determines a time ratio of one phase excitation to two-phase excitation, e.g., the one phase excitation time and the two-phase excitation time are fixed to the same period. Such a conventional motor drive device has various problems as will be explained below.
In order to prevent, for instance, the facsimile machine's image scanning from being affected by the rotating motor, it is required to reduce mechanical vibrations of the motor as much as possible. Generally, the mechanical vibrations of the motor are determined by a mechanical structure of the facsimile machine, in which the motor is incorporated, and the motor excitation period.
The mechanical structure of the facsimile machine constitutes a vibration structure. The mechanical structure of the facsimile machine has a national frequency which is determined by inertia of the stepping motor itself, inertia of rollers driven by the stepping motor to feed a paper in a sub-scanning direction, friction between the rollers and a document or a recording sheet and characteristics of gears or belts connecting the stepping motor with the rollers. When the natural frequency coincides with a drive frequency of the stepping motor or n times thereof, the mechanical structure of the facsimile machine resonates with the stepping motor.
Therefore, even if the use of a motor drive mechanism having an excitation time ratio of 1 to 1 (one phase excitation time to two-phase excitation time), for example, in a certain facsimile machine does not result in significant vibrations, the use of the same motor drive mechanism in a different facsimile machine might result in considerable vibrations because of the mechanical structure of the latter facsimile machine.
In a conventional motor drive mechanism, the one phase/two-phase excitation time ratio is generally determined and fixed by a hardware of the motor drive mechanism. Thus, it is impossible to adjust the excitation time ratio when a single motor drive device is used for different kinds of mechanical structures and large vibrations are produced in a certain type of facsimile machine.
The vibrations of the mechanical structure of the facsimile machine raise two problems. One is that the facsimile machine produces noises which in turn make users displeased. The other is that the vibrations of the mechanical structure are unavoidably transmitted to the document and image sensors. Because of these vibrations, the facsimile machine cannot clearly read an image. In another case, the vibrations may propagate to a thermal print head and a thermosensitive paper so that the facsimile machine cannot perform clear image recording.
Particularly, the one-two phase excitation of the motor is mostly performed when the facsimile machine is set to the fine mode which requires better image quality than usual. Accordingly, the image quality degradation due to the vibrations during the one-two phase excitation is a serious problem.
To prevent the vibrations of the motor, conventionally a wide variety of motor drive devices are prepared. Specifically, the motor drive devices having excitation ratios suitable for various kinds of facsimile machines are prepared and a particular motor drive device is installed in a particular facsimile machine. However, this lowers the manufacturing efficiency of the facsimile machines and raises cost.
The above problem also occurs in other equipment such as scanners and printers.
When the facsimile machine finishes the scanning of one line of a document in a main scanning direction, the motor drive device conducts a predetermined step drive of a document feed stepping motor in response to a motor drive instruction from a CPU. Also, upon the completion of one line recording on a thermosensitive paper, the motor drive device conducts a predetermined step drive of a thermosensitive paper feed stepping motor in response to a motor drive instruction from a CPU.
However, when the motor drive device drives the stepping motor immediately after it receives the motor drive instruction, the mechanical structure of the facsimile machine sometimes resonates with the stepping motor, as mentioned earlier. It is known through experience that driving the stepping motor with a certain time delay after the motor drive instruction can eliminate the problem of resonance.
The above-mentioned certain time delay has another meaning: even if the mechanical structure vibrations do not occur, microscopic vibrations take place in the document. This will be explained with FIG. 14 of the accompanying drawings. FIG. 14(a) shows a read instruction signal from a CPU, FIG. 14(b) shows a motor drive instruction signal and FIG. 4(c) shows an excitation phase. In the illustration, numerals a1 and a2 indicate charging periods of an image sensor (CCD), respectively. The image sensor outputs image data, which has been read during a previous charging period, in response to the read instruction signal. The stepping motor ideally feeds the document as shown in FIG. 14(d). However, various factors such as the starting property (performance) of the stepping motor itself, mechanical transmission delay between the stepping motor and the rollers and friction and slippage between the rollers and the document cause the microscopic vibrations d1 and d2, as shown in FIG. 14(e), when the stepping motor feeds the document.
It is understood from FIGS. 14(b) and 14(c) that the motor drive device changes the excitation phase as it receives the motor drive command. In other words, the motor drive device carries out the one step drive of the stepping motor upon receiving the motor drive command. It is understood from FIGS. 14(a) and 14(e) that the microscopic vibrations d1 and d2 occur during the charging periods a1 and a2 of the image sensor. This implies that the image sensor outputs image data which is derived from a vibrating document. It is impossible for a printer of the facsimile machine to reproduce a clear image from such image data.
In FIG. 14(f), the motor drive device changes the excitation phase (or performs one step drive of the stepping motor) with a predetermined time delay Td after the motor drive command. In this case, the document moves as shown in FIG. 14(g) and microscopic vibrations f1 and f2 arise outside the image sensor charging periods a1 and a2. This implies that the image sensor reads the document with no microscopic vibrations and therefore the printer of the facsimile machine can reproduce a clear image.
The delay time Td of the conventional motor drive device is determined by experiments and a timer (hardware) for embodying the delay time Td is incorporated in the motor drive device. However, since this motor drive device is designed for a facsimile machine having a particular mechanical structure, the application of this motor drive device to a facsimile machine having another mechanical structure may not work satisfactorily. This is because the delay time Td of one facsimile machine is generally not equal to that of another facsimile machine.
Conventionally, therefore, the motor drive device having the same numbers as the types of the facsimile machine are prepared and respective motor drive devices are incorporated in corresponding facsimile devices, as mentioned before. This raises cost and lowers the manufacturing efficiency.